In the relatively recent past, there has been an increasing desire to replace metallic parts with parts made from polymer materials, particularly high performance thermoplastic polymers. The need extends to almost a limitless variety of different applications and fields. For instance, parts made from thermoplastic polymers are used in consumer appliance products, industrial processes, in all sorts of transportation vehicles, and the like. For example, there is an increasing demand to replace metal parts in the automotive field with parts made from thermoplastic polymers. The parts made from the thermoplastic polymers, for instance, may be used on the exterior of the vehicle or on the interior of the vehicle.
When used in automotive applications, for instance, the parts made from thermoplastic polymers typically are required to have an aesthetic appearance in order to further increase the appeal of the vehicle. Thus, in many applications, after being molded, the parts are then painted or otherwise decorated in order to color coordinate with the environment in which they are used. Unfortunately, painting the plastic parts represents a substantial expense in the production of the part. Having to paint the part also increases production time and can significantly add capital expense to the facilities in which the parts are produced.
In view of the above, those skilled in the art have attempted to add coloring agents to thermoplastic polymers in order to forego having to paint the molded parts. In one embodiment, for instance, metallic pigments are added to thermoplastic compounds to provide molded parts having a metallic appearance. During injection molding of polymer articles, however, various surface defects can appear during production of the part. For example, certain part geometries, mold layouts, the location of gates, and molding conditions can cause various drawbacks including increased gloss loss on weathering and visual defects affecting part aesthetics. One particular reoccurring problem is known as “gate blush” which results in irregular dark patterns or a dull or discolored area near the gate location of an injection mold. Another reoccurring problem is the formation of flow lines in the finished part formed at the meeting of two flow fronts when the mold configuration includes more than one gate. These flow lines are sometimes referred to in the art as “knit-lines”.
Knit-lines can be eliminated in many applications by having a mold configuration that only includes a single gate or point of entry for molten thermoplastic material into the mold. Attempts to eliminate gate blush, on the other hand, have generally been unsuccessful in many different molding configurations. Controlling molding conditions, such as injection speed, or moving the gate to a particular location on the mold has not been found to consistently eliminate gate blush. Also conventional is the use of cold slug passages, which are small protrusions located close to the gate. Despite the presence of cold slug passages, gate blush continues to occur. Reducing the fill rate of the mold has also not consistently eliminated gate blush and also results in a slow cycle time. Those skilled in the art simply do not have a good understanding of why gate blush occurs in certain circumstances and therefore have been unable to completely solve the problem.